My career goal is to become an independent investigator that contributes to the understanding of tendon biology and develops improved strategies for tendon repair. My research training started as an engineer who focused on developing therapies that return tendon back to normal mechanical function. I realized that an incomplete knowledge of the cellular mechanisms that lead to improved repair was preventing me from developing an effective therapeutic strategy. I decided that I needed to improve my understanding of the origin of tendon progenitors, the markers that define their differentiation state, and the signaling pathways that regulate their differentiation prior to developing novel repair strategies. My mentoring team, consisting of my primary mentor Dr. David Rowe and other skilled musculoskeletal biologists and engineers at the University of Connecticut Health Center along with an outside tendon developmental biologist, provide me with an exceptional environment to investigate these questions and develop the necessary skills to contribute to the field as an independent investigator. We have developed a repertoire of transgenic lineage tracing and GFP reporter mice that identifies resident tendon progenitor populations and showcases the heterogeneity of cells in the tendon midsubstance that was once considered quite homogeneous. This application lays out a strategy to characterize the transciptomic and proteomic profile of resident tendon progenitors as they differentiate during normal processes of tendon growth and natural healing following injury. The central hypothesis is that the molecular mechanisms that drive tendon progenitor differentiation during growth are crucial to improving tendon repair in the adult. Therefore, adult progenitors that mimic the growth differentiation profile during repair will lead to improved mechanical outcome. We have identified resident tendon progenitors in an inducible Cre model for alpha smooth muscle actin (aSMA-CreERT2) that contribute to scleraxis-expressing (ScxGFP) fibroblasts in the tendon midsubstance during growth and are also the main contributors to tendon healing from the paratenon in the adult. The SMACre model will be used in combination with four other GFP reporter mouse models that identify subpopulations within this lineage in the tendon midsubstance. We will define the expression profiles that delineate these subpopulations to map the heterogeneity of the tendon midsubstance. To better define the signaling pathways that regulate differentiation of these resident progenitors, we will begin by knocking out TGF signaling in these cells to determine how this important pathway regulates normal cell turnover during growth and healing. Finally, using the expression profiles and markers defined in earlier studies, we will isolate analogous resident progenitors from human tendon tissue. The reparative potential of these human sources will be compared to their SMACre counterparts in a common test platform in the patellar tendon defect of immunodeficient NSG mice. If successful, this proposal will 1) help characterize the resident tendon progenitors that respond to injury, 2) help isolate progenitor cel sources conducive for tendon repair, and 3) provide biological success criteria for tenogenic differentiation. Finally, this research strategy under the guidance of my mentoring team will give me the experience needed to jump start a career in tendon biology and tissue engineering.